Here's a place where I can post my thoughts on new papers, provide updates on my projects, and post info that will eventually be on my website The Theropod Database - http://theropoddatabase.com/ . It will center on theropods, but may delve into other topics as well such as phylogenetics.

Wednesday, January 6, 2016

Chiappeavis is just another Pengornis

The last Mesozoic dinosaur named in 2015 is Chiappeavis magnapremaxillo (O'Connor et al., 2015b), electronically published on December 31st. First of all, what a stupid species name. Second, examination of the supposed diagnostic characters shows that like Parapengornis and IVPP V18632, it's just another example of Pengornis houi. Which is a shame because Luis Chiappe really deserves a bird named after him. So before anyone names a 'Chiappeornis', let's make sure it's a really distinctive specimen, okay? It's also disappointing because O'Connor has an entire section of her thesis lamenting the fact so many enantiornithines are named without adequate diagnoses. But here she's doing that herself (and yes, the contributions credit her with writing the manuscript). I'll also note the paper lacks a measurement table and that Pengornithidae can be paraphyletic in their analysis depending on their assumptions (fig. S1E). Anyway, let's look at the evidence...

Several characters were listed as supposedly diagnostic. The premaxilla was said to have a larger body and convex ventral margin, but the convex element has far too long of a ventral margin to be a premaxilla, so is more likely an incomplete maxilla with the base of the ascending process. The actual right premaxilla is then just the small anterodorsal portion right below the partially preserved left premaxilla, and has no visible ventral margin.

The posterodorsal premaxillary process was claimed to be longer than other pengornithids, almost reaching the frontals. Yet this bone is highly abraded just posterior to figure 2A's 'l pm' label, so that this posterior portion could easily belong to the right nasal instead. This is bolstered by the fact it's the same width as the left nasal and that the right nasal is otherwise missing. These cranial reinterpretations also make far more sense as they match the morphology and preservation of other pengornithids, which would be expected as the postcrania are nearly identical.

The synsacrum has eight vertebrae (as in IVPP V18632; Parapengornis' is only partly preserved), while the authors claim Pengornis' holotype has seven. Yet the description of the latter says only that seven are visible, but that the anterior end is covered, and indeed there could easily be another one beneath the ilium. STM 29-15 (a Jiufotang pengornithid briefly described by O'Connor et al., 2015a) only has seven but is obviously younger based on its unfused sterna.

The "median trabeculae" of Chiappeavis are said to have concave lateral margins, but no such structure exists, so the authors clearly meant the posterolateral processes. Their slight lateral concavity is also seen in Parapengornis' holotype though not in STM 29-15 and perhaps IVPP V18632 (difficult to determine from the low resolution figure of the latter), while Pengornis' holotype doesn't preserve the sternum.[Edit] O'Connor cordially informed me she meant the posteromedian process, so incorrectly pluralized the term instead of my assumption that she used the wrong positional adjective ("median trabeculae is totally a valid term MORON - they fuse into xiphial region. check Baumel bitch" as per O'Connor pers. comm., 1-16-2016). However, concave
lateral edges on the posteromedian process are also present in IVPP V18632,
absent in STM 29-15, and unknown in both Pengornis and Parapengornis.
So this doesn't support Chiappeavis' validity either.

The posteromedian angle of the sternum is said to be narrow, but while its 53 degree angle is a bit less than IVPP V18632's (at 68) or STM 29-15's (at 66), Parapengornis' holotype could have an identical angle if complete, and again Pengornis' holotype doesn't preserve the element.

Finally, the authors claim the proximal articular surface of the tibia is laterodistally inclined, but this is only true of the left tibia, with the right tibia having a right angle between the surface and the long axis of the bone. Furthermore, Pengornis' holotype and IVPP V18632 both have inclined surfaces, though Parapengornis' holotype lacks them. Given the variation in Chiappeavis' holotype, the variation is likely due to perspective or taphonomy.

Tibiotarsi of (left to right; flipped if necessary so that lateral is to the left) both of Pengornis houi holotype IVPP V15336 (after Zhou et al., 2008), right of IVPP V18632 (after Hu et al., 2014; left is in medial view), both of Parapengornis eurycaudatus holotype IVPP V18687 (after Hu et al., 2015), left of STM 29-15 (after O'Connor et al., 2015a; right is fragmented); and Chiappeavis magnapremaxillo holotype STM 29-11 (aftter O'Connor et al., 2015b). Angle of proximal edge indicated in green; note that Chiappeavis' angles differ and that Pengornis' are both ventrolaterally angled.

Besides the characters listed in the diagnosis, O'Connor et al. note the short anterior cervicals are like the Parapengornis holotype but unlike Pengornis' holotype. As the latter is larger than the others, this may support ontogenetic cervical elongation.

They also correctly note the long pygostyle is like Pengornis' holotype, but unlike IVPP V18632 or Parapengornis' holotype.

Finally, the short metatarsal I is said to be like Pengornis' holotype, and is additionally like IVPP V18632 but unlike Parapengornis' holotype or STM 29-15.

Thus the only real difference from Pengornis' holotype is the shorter anterior cervicals, and while there are a few differences from other pengornithid specimens (laterally concave posterolateralposteromedian sternal processes unlike STM 29-15 and (?)IVPP V18632; narrower posterior sternal angle than STM 29-15 and IVPP V18632; long pygostyle unlike STM 29-15, IVPP V18632 and Parapengornis' type; short metatarsal I unlike STM 29-15 and Parapengornis' type), there's no pattern of character distribution that would suggest separate pengornithid species (e.g. Parapengornis shares the cervical length, posterolateral process concavity and probably sternal angle, while IVPP V18632 is different in cervical length but shares metatarsal I length). This is illustrated in the following table, showing the distribution of all real differences proposed to diagnose Jiufotang pengornithid taxa (known in more than two specimens) that aren't obviously ontogenetic-

Note the lack of a pattern unrelated to size. Indeed, all but E, F and I correlate with size, and two of those conflict with eachthe other. It's true that size is not fully correlated with osteological development in this series (e.g. STM 29-15 has unfused sternals unlike the slightly smaller IVPP V18632), but that's true of other taxa as well (e.g. Anchiornis, Archaeopteryx, Shenzhouraptor, Sapeornis, Archaeorhynchus). So it's just like with Archaeopteryx and Microraptor- all specimens show uncorrelated differences, thus either every specimen is its own species or as far as we can tell they're a single species that shows variation. Based on this I conclude all Jiufotang pengornithids should still be still retained in Pengornis houi.

9 comments:

So you're proposing that both Chiappeavis AND Parapengornis may be junior synonyms of Pengornis? If so, and if Pengornithidae is paraphyletic (as in Fig. S1E), then it suggests a loss of the aerodynamic tail fan (associated with rectricial bulbs as the "rectricial complex") could have occurred just once at the base of the Enantiornithes.

Yup, I believe both are synonyms (though Parapengornis has more valid differences from the Pengornis holotype at least). And yes, if S1E is correct, the loss of the tail fan could have happened just once. The same goes for S1A, where pengornithids are monophyletic, but basal within Enantiornithes. But the differences between the equally parsimonious trees in S1A and S1B show that like mine and Cau's matrices, O'Connor's matrix doesn't strongly support many relationships within Enantiornithes. So I wouldn't put much trust into any given topology or distribution of any given character like retrical morphology.

Eopengornis has no tail fan (and by implication, no rectricial complex), so to me Fig S1A (recovering a monophyletic Pengornithidae) would imply that this feature was lost *twice* in Enantiornithes: (1) in the Eopengornis line of Pengornithidae, (2) the clade comprising the rest of the Enantiornithes (= sister clade to Pengornithidae in S1A).

O'Connor et al. make the point that the 'rectricial complex' morphology comprises a number of features, in both the skeleton (pygostyle morphology) and integument (rectricial morphology and arrangement). (There's also the rectricial bulbs, but they're invisible in the fossil record.) So should these characters be treated as correlated?

We do have probable preserved retricial bulbs in the enantiornithine Feitianius, which besides the long paired retrices of most enantiornithines, has vaned feathers over three times as long as the pygostyle. Do those constitute a tail fan? The pygostyle's pretty standard for enantiornithines. But no, I'd say don't treat characters as correlated unless one logically forces the other.

I have always found it strikingly odd that all Chinese Jehol and related birds comprise monophyletic genera - in clear contradiction to what one see in the real world. So i look forward to the time when enantiornithines etc are revised to actually document the real generic diversity and identify the multiple synonyms that there are now.

The first is cultural. We don't see genera in the real world; genera don't exist outside of taxonomists' heads. Therefore, dinosaur paleontology has a long tradition of using the genus as the unit of biodiversity; genera with two or more species are typically split after a few decades or see their species synonymized. Because genus names are unique, they are used to refer to species; "Tyrannosaurus" is talked about rather than "T. rex". Mickey's complaint isn't that the pengornithid species shouldn't be split into as many genera; it's that they should all be referred to a single species.

The second is that the fossil record is neither all that complete nor all that well known; a random sample of a few species will likely not turn up species that an ornitho-neontologist would classify into the same genus.